It’s still possible to use the full resolution Strava Heatmap in JOSM by copying some keys from your cookie data - this is discussed in the link provided by Adamant1.

This is quite tedious and also requires you to redo the process about once a month when the heatmap updates, but it’s a functional workaround. I’m not aware of any changes to Heatmap terms of use so I think we’re still allowed to use it (or at least still not explicitly disallowed..). Only change is we now also need an individual Strava account.